The present application relates to pressure actuated electrical switches. More particularly, methods and devices are provided for arming successive explosive charges upon actuation of a switch by pressure waves from previous detonations.
In oil and gas exploration and production operations, well bores are drilled into the ground to gain access to subsurface hydrocarbon-bearing formations or reservoirs. Well bores are typically lined with steel tubing, known as casing or liner, to provide the wellbore with a stable, permanent barrier. This casing is often secured to the wellbore by cement that is pumped into the annulus between the outside diameter of the casing and the inside diameter of the wellbore wall.
While the casing stabilizes the wellbore wall, it also seals the fluids within the earth strata. Thus, the casing must be opened or perforated to allow the inflow of hydrocarbons into the casing for extraction. To selectively open the casing to such fluid flow, the casing is often penetrated in the region of a fluid production zone by shaped or oriented charge explosives, which when detonated, penetrate the casing creating perforations through which fluid in the formation may flow. The tubular tool section that carries these explosives is often referred to as a “perforation gun” or more simply as a “gun.”
Often, it is desired to perforate a casing at multiple locations to access hydrocarbons residing in multiple subterranean zones. To accomplish these perforations in the casing, charged explosives are typically used to penetrate the casing. The charged explosives are usually delivered by way of a tubular gun, typically referred to as a perforation gun.
Inadvertent activation of such explosives pose a potential hazard to personnel. Additionally, inadvertent firing of an perforation gun or self-detonation thereof while the gun is being positioned or retrieved can damage the wellbore, such as perforating the casing at an undesired depth. Moreover, explosives that fail to fire or for some reason are not fired must be retrieved from the wellbore in its unfired condition, creating a potential hazard to both personnel and the wellbore, not to mention the resulting lost operation time. Further complicating operation of these perforation guns is the requirement of creating multiple perforations at multiple depths. Plus, each perforation may require activation of a different number or set of explosives.
Accordingly, a variety of switching mechanisms have been designed to control activation of multiple explosives. U.S. Pat. No. 4,457,383 describes one example of a conventional switching unit. In devices of this type, a plurality of blasting cap-perforating element assemblages are spaced apart along the length of a perforation gun. The assemblage that is furthest downhole is typically armed, while the other successive assemblages are disarmed. When the armed assemblage is fired, the next adjacent assemblage closest to the discharged assemblage is armed through the use of a mechanically operated switch.
Thus, conventional switching units have been described that arm a subsequent charge upon a first charge being fired while, at the same time, disconnecting the firing mechanism from the first charge. This result is accomplished because the hot wire side of the firing circuit includes a switch for each initiator-perforating element assemblage which completes a bypass circuit to the next upper assemblage while disarming its associated assemblage. Upon firing the lowermost assemblage, the switch of the next upper assemblage is manipulated to arm its associated blasting cap. Firing of charges carried by the perforation gun may in this fashion proceed from the bottom of the gun toward the top of the perforation gun.
Generally, conventional switches of this type involve a “bullet” that is thrust axially in the switching mechanism by the explosive force of a preceding charge. The axial movement of the bullet is intended to disconnect the arming of the previously activated charge and at the same time, or immediately thereafter, engage the arming mechanism of a subsequent explosive charge. The disadvantages of such conventional switches are numerous, including malfunctions involving the bullet failing to move the desired axial distance or being propelled farther than its desired distance. Where the bullet is insufficiently propelled the desired axial distance, the bullet will not properly engage the arming mechanism of the subsequent explosive charge and thus fail to engage the subsequent explosive charge for activation. Additionally, the bullet, by failing to be propelled the desired axial distance will also fail to disconnect with the previously activated explosive charge. Where the bullet is propelled past its intended destination, on the other hand, it may fail to adequately engage the subsequent arming mechanism.
Additionally, because of the design limitations of conventional switching units, such switching units only effectively operate within a narrow range of temperatures and pressures. Therefore, such conventional switches frequently fail to operate outside of the narrow range of conditions for which they are designed. Moreover, fluid contamination in portions of the perforation gun may contaminate the arming mechanisms or switching units so as to prevent the proper operation thereof. For example, fluid within the above described prior art switch often has the effect of inhibiting the pressure mechanism used to propel the “bullet.” Without sufficient force to propel the bullet, the mechanism will fail as described above.
Therefore, improved pressure actuated electrical switching devices are needed to address one or more disadvantages of the prior art.